Arrhenius Definition. Chapter 15 Acids and Bases. Brønsted-Lowry Concept. Brønsted-Lowry Concept. Conjugate Acid-Base Pairs

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1 John W. Moore Conrad L. Stanitski Peter C. Jurs Chapter 15 Acids and Bases Arrhenius Definition Arrhenius: any substance which ionizes in water to produce: Protons is an Acid. ydroxide ions is a Base. Better version of the Arrhenius definition: Acid: ydronium ions ( 3 + ) in water are acidic. Base: ydroxide ions ( - ) in water are basic. Stephen C. oster Mississippi State University So, why is N 3 (aq) basic? Brønsted-Lowry Concept An alternative definition: Brønsted-Lowry Acid = Proton donor Brønsted-Lowry Base = Proton acceptor Works for non-aqueous solutions and explains why N 3 is basic: N 3 (g) + 2 (l) Base: + acceptor Acid: + donor N 4+ (aq) + - (aq) Brønsted-Lowry Concept Strong acids and bases almost completely ionize. N 3 (aq) + 2 (l) 3 + (aq) + N 3- (aq) Weak acids and bases do not fully ionize. (aq) + 2 (l) 2 (l) + N 3 (aq) Acid: + donor Base: + acceptor 3 + (aq) + - (aq) N 4+ (aq) + - (aq) Acid: + donor Base: + acceptor Note: the products are a new acid and base pair. Water s Role as Acid or Base Water acts as a base when an acid dissolves in water: Br(aq) + 2 (l) 3 + (aq) + Br - (aq) acid base acid base But water acts as an acid for some bases: 2 (l) + N 3 (aq) N 4+ (aq) + - (aq) acid base acid base Water is amphiprotic - it can donate or accept a proton (act as acid or base). Conjugate Acid-Base Pairs Molecules or ions related by the loss/gain of one +. Conjugate Acid 3 + Conjugate Base 2 C 3 C C 3 C - donate + N 4 + N 3 accept + 2 S 4 S 4 - S 4 - S 4 Cl Cl - N 4+ and N are not conjugate, conversion requires

2 Conjugate Acid-Base Pairs Identify the base conjugate to (aq) and the acid conjugate to C 3- (aq) Acid: + donor C C 3 Base: + acceptor Conjugate Base Conjugate Acid ( C 3 is the base conjugate to C 3- ) Relative Strength of Acids & Bases Strong acids are better + donors than weak acids Strong bases are better + acceptors than weak bases Stronger acids have weaker conjugate bases. Weaker acids have stronger conjugate bases. Strong acid conjugate base ully ionized, reverse reaction essentially does not occur. The conjugate base is weak. Weak acid conjugate base Weakly ionized, reverse reaction readily occurs. The conjugate base is strong. Relative Strength of Acids & Bases strong Acid strength increasing extremely weak Conjugate acid Conjugate base 2 S 4 S - 4 Br Br - Cl Cl - N 3 N S 3 S - 3 S - 4 S 4 3 P 4 2 P C 3 C C 3 C - 2 S S - 2 P - 4 P 4 N + 4 N 3 C - 3 C C 4 C - 3 extremely weak Base strength increasing strong Relative Strength of Acids & Bases Problem Is the following aqueous reaction product or reactant favored? N is a stronger acid than N 4+. (N 3 is a stronger base than - ) has greater tendency to ionize than N 4+. (N 3 more readily accepts + than - ) Reactant avored N 3 + Acid strength increasing Conj acid. Conj. base 2 S 4 S - 4 Br Br - Cl Cl - - N 4 + N C 4 C - 3 Base strength increasing Carboxylic Acids and Amines rganic acids Contain the carboxylic acid functional group: Carboxylic Acids and Amines The carboxylic acid is acidic (will ionize). All other protons are non acidic: C C C 3 C - is very electronegative. withdraws e - from the bond (weakens -). Makes it easier for + to leave. nce formed, the anion is stabilized by resonance: C 3 C - - C 3 C 2

3 Carboxylic Acids and Amines Amine group: R N 2, R NR or R NR 2 R = any hydrocarbon. Lone-pair on N accepts a proton (like N 3 ). Autoionization of Water Two water molecules can react to form ions. Autoionization occurs: 2 (l) + 2 (l) Base Acid 3 + (aq) + - (aq) Acid Base eavily reactant favored. nly a very small fraction is ionized: ionization constant for water K w = [ 3 + ] [ - ] = (1.0 x 10-7 )(1.0 x 10-7 ) = 1.0 x (at 25 C) [ 2 ] 2 is omitted Ionization Constant for Water K w, like all equilibrium constants, is T-dependent. T = 25 C (77 ) is usually used as the standard T. T ( C) K w x x x x x x Autoionization of Water 3 + and - are present in all aqueous solutions. Neutral solution. Pure water (@ 25 C): [ 3 + ] = 10-7 M = [ - ] Acidic solution If acid is added to water: [ 3 + ] is increased, disturbing the equilibrium: Equilibrium shifts to remove [ 3 + ] (and [ - ]). Equilibrium is reestablished: [ 3 + ] > 10-7 M > [ - ] [ 3 + ][ - ] = K w = 1.0 x at 25 C. Autoionization of Water Basic solution. If base is added to water. Equilibrium shifts to remove [ - ] (and [ 3 + ]). New equilibrium: [ 3 + ] < 10-7 M < [ - ]. [ 3 + ][ - ] = K w = 1.0 x at 25 C. Autoionization of Water Example Calculate the hydronium and hydroxide ion concentrations at 25 C in a 6.0 M aqueous sodium hydroxide solution. K w = [ 3 + ][ - ] = 1.0 x Na (aq) is strong (100% ionized) so [ - ] = 6.0 M. [ 3 + ](6.0) = 1.0 x [ 3 + ] = 1.7 x M [ - ] = 6.0 M 3

4 The p scale The p scale Acid concentration can vary over a very large range. A logarithmic scale is more convenient: p = log 10 [ 3 + ] At 25 C a neutral aqueous solution has: p = log 10 [1.0 x 10-7 ] = ( 7.00) = 7.00 Acidic solutions: p < 7.00 (lower p = more acidic). Basic solutions: p > 7.00 (higher p = more basic). Examples Lemon juice: weak acid: [ 3 + ] = 1 x 10-2 M p = ( 2.0) = 2.0 [ 3 + ] = 2.4 x 10-5 M p = ( 4.62) = 4.62 log 10 (1 x 10 -x ) = -x 2 sig. digits - digits after the decimal point are significant: -4 is the power of 10 for the number (0.24 x 10-4 ). The p scale Base concentrations: p = log 10 [ - ] A neutral solution (25 C) has: p = log 10 [1.0 x 10-7 ] = ( 7.00) = 7.00 Since K w = [ 3 + ][ - ] = 1.0 x log(k W )= log[ 3 + ] + ( log[ - ]) = log(1.0 x ) pk w = p + p = (Valid in all aq. solns. at 25 C: acidic, neutral or basic) p Calculations Given two aqueous solutions (25 C). Solution A: [ - ] = 4.3 x 10-4 M, Solution B: [ 3 + ] = 7.5 x 10-9 M. Which has the higher p? Which is more acidic? Solution A: p = log[ - ] = 3.37 p + p = pk w = p = Solution B: p = log[ 3 + ] = 8.12 A has higher p, B is more acidic Measuring p 3 + concentrations can be measured with an: Electronic p meter: fast and accurate. preferred method. Acid-base Indicator: substance changes color over a small p range. may have multiple colors (e.g. bromthymol blue). one color may be colorless (e.g. phenolphthalein). cheap and convenient. Ionization Constants of Acids and Bases When an acid ionizes in water: A(aq) + 2 (l) 3 + (aq) + A - (aq) The acid ionization constant is used to report the degree of ionization: K a = [A - ][ 3 + ] [A] Strong acids have large K a values. Weak acid have small K a values. (Water omitted, as usual) 4

5 Base Ionization Constants or a base in water: B(aq) + 2 (l) The base ionization constant, K b, is: If the base is an anion: A - (aq) + 2 (l) K b = [B+ ][ - ] [B] K b = [A][- ] [A - ] B + (aq) + - (aq) A(aq) + - (aq) Values of Acid and Base Ionization Constants See Table 15.2: ionization constants for common acids and their conjugate bases. Strong acid K a are hard to measure. They are very large, 100% ionized. Assume they are fully ionized. Larger K a = stronger acid. Strong base K b are hard to measure Larger K b = stronger base. K a Values for Polyprotic Acids Some acids can donate more than one + : K a Values for Polyprotic Acids Phosphoric acid ( 3 P 4 ) has three acidic protons: ormula Name Acidic s 2 S ydrosulfuric Acid 2 3 P 4 Phosphoric Acid 3 2 C 3 Carbonic Acid 2 C-C xalic acid 2 C 3 5 (C) 3 Citric acid 3 Each + ionization has a different K a. The 1 st proton is easiest to remove. The 2 nd is harder, etc. 3 P 4 (aq) + 2 (l) 2 P 4- (aq) + 2 (l) P 4 (aq) + 2 (l) 3 + (aq) + 2 P 4- (aq) K a = 7.5 x (aq) + P 4 (aq) K a = 6.2 x (aq) + P 3-4 (aq) K a = 3.6 x harder to remove Molecular Structure and Acid Strength What makes a strong acid? A weak A bond, so + can be easily removed! Strength of xoacids --Z Acid Bonding EN Z K a Consider the binary acids, Cl, Br, I. X Bond Energy (kj) K a weak acid x 10-4 Cl x 10 7 Br x 10 8 I x strong acids smaller bond energy larger acid strength hypochlorous acid Cl --Cl x 10-8 hypobromous acid Br --Br x 10-9 hypoiodous acid I --I x Weaker - bond Stronger acid. If Z is electronegative it will pull e - to it: Z Larger EN Z = weaker bond (stronger acid). 5

6 Strength of xoacids --Z with Z = group of atoms. Acid Bonding K a perchloric acid Cl 4 --Cl 3 1 x 10 8 chloric acid Cl 3 --Cl 2 1 x 10 3 chlorous acid Cl 2 --Cl 1 x 10-2 hypochlorous acid Cl --Cl 4 x 10-8 Stronger acid = weaker - bond. If Z is: More e - withdrawing = weaker bond = stronger acid. More atoms = more e - withdrawing. Example: K a from p Lactic acid is monoprotic. The p of a M solution was 2.43 at 25 C. Determine K a for this acid. Using the p information: - log [ 3 + ] = 2.43 [ 3 + ] = = M These hydronium ions are produced by: A(aq) + 2 (l) 2 (l) + 2 (l) 3 + (aq) + A - (aq) and from the autoionization of water: 3 + (aq) + - (aq) A(aq) + 2 (l) = (0.0037)(0.0037) = 1.4 x 10-4 ( ) 3 + (aq) + A - (aq) [ ] initial x (water autoionization) [ ] change [ ] equil * K a = [ 3 + ][A - ] [A] *Ignore the water contribution because it is so small. Example: p from K a Determine the p of a M propanoic acid solution at 25 C. K a = 1.4 x What % of acid is ionized? C 2 5 C + 2 (l) 3 + (aq) + C 2 5 C - (aq) r A A - K a = [ 3 + ][A - ] = 1.4 x 10-5 [A] Determine the p of M propanoic acid soln. at 25 C. K a = 1.4 x10-5. What % of acid is ionized? A(aq) + 2 (l) 3 + (aq) + A - (aq) [ ] initial * 0 [ ] change -x +x +x [ ] equil x x* x [ 3 + ][A - ] K a = 1.4 x 10-5 = = [A] *Ignore the water contribution (0.100 x) x << 0.1 (K a is very small) x x = (1.4 x 10-5 )(0.100) = M (x << M; the approximation is good) p = -log( ) = 2.93 x ( ) %-ionized = 100% = 1.18 %

7 Determine [ - ] and p of a M dimethylamine solution at 25 C. K b = 5.9 x (C 3 ) 2 N + 2 (l) (C 3 ) 2 N 2+ (aq) + - (aq) B(aq) + 2 (l) B + (aq) + - (aq) [ ] initial * [ ] change -x +x +x [ ] equilib x x x* K b = 5.9 x 10-4 = [B + ][ - ] [B] x = 2 (0.060 x) *Ignore water. x = (5.9 x 10-4 )(0.060) = M = [ - ] p = 2.23 p = = x [B]/10, the approximation of x << [B] is marginal Check the exact solution x 10-4 = x = (5.90 x 10-4) (0.060 x) x 10-4 x 3.54 x 10-5 = 0 Quadratic solution: x = M p = 2.25 p = The approximate method gave p = 2.23 In general, approx. method is good if x < [ ]/20 Relationship between K a and K b values or an acid-base conjugate pair: A and A - K a x K b = [ 3 + ][A - ] [A] [A][ - ] [A - ] Phenol, C 6 5, is a weak acid, K a = 1.3 x at 25 C. Calculate K b for the phenolate ion C K a x K b = 1.0 x x 10 K b = -14 = 7.7 x x = [ 3 + ][ - ] = K w Acid Base Reactions of Salts Salt = ionic compound formed in acid + base reaction. X(aq) + M(aq) MX(aq) + 2 (l) acid base salt Strong acid + strong base: Cl(aq) + Na(aq) NaCl(aq) + 2 (l) 3 + (aq) + Cl - (aq) + Na + (aq) + - (aq) Na + (aq) + Cl - (aq) (l) Net ionic equation: 3 + (aq) + - (aq) 2 2 (l) Salts of Strong Bases and Strong Acids 3 + (aq) + - (aq) 2 (l) + 2 (l) Acid Base Base Acid Na + and Cl - are spectators appear as reactants and products. Net products are water: weak acid and weak base. inal solution has p = 7. Strong acid + strong base salt and water. Equal moles generate a neutral solution. 7

8 Salts of Strong Bases and Weak Acids Weak acid + strong base is more complex. C 3 C(aq) + Na(aq) C 3 CNa(aq) + 2 (l) Weak acids partially ionize in solution. Do not dissociate the acid on the reactant side. C 3 C(aq) + Na + (aq) + - (aq) Na + (aq) + C 3 C - (aq) + 2 Salts of Strong Bases and Weak Acids C 3 C(aq) + - (aq) Acetate ions form: acetate is a weak base (much stronger than water). solution is basic (p > 7.0). C 3 C - (aq) + 2 (l) C 3 C - (aq) + 2 (l) Acid Base Base Acid C 3 C(aq) + - (aq) Net ionic: C 3 C(aq) + - (aq) C 3 C (l) A hydrolysis reaction water is broken apart. Salts of Strong Bases and Weak Acids or a weak acid + strong base, p depends on K b. Larger K b = stronger base Example What is the p of 1.50 M Na 2 C 3 (aq)? K b (C 3 ) = 2.1 x Na + is the conjugate acid of Na (strong base). It remains 100% ionized. Na + has no effect on p. C 3 is the conjugate base of C 3- (weak acid). ydrolysis: C C 3- and - Changes the p. p of a Salt Solution What is the p of a 1.50 M Na 2 C 3 (aq)? K b = 2.1 x 10-4 C 3 (aq) + 2 (l) C 3- (aq) + - (aq) [ ] initial [ ] change - x +x +x [ ] equil x x x K b = 2.1 x 10-4 = [C 3- ][ - ] = [C 3 ] (1.50 x) x = 1.77 x 10-2 p = log(1.77 x 10-2 ) = 1.75 p = = Salts of Weak Bases and Strong Acids Similar arguments. N 3 (aq) + Cl(aq) N 4+ (aq) + Cl - (aq) N 3 (aq) (aq) + Cl - (aq) N 4+ (aq) + Cl - (aq) + 2 (l) Net ionic: N 3 (aq) (aq) N 4+ (aq) + 2 (l) An equimolar reaction produces an acidic solution: N 4+ (aq) + 2 (l) N 3 (aq) (aq) The final p depends on K a : Larger K a = more acidic p of a Salt Solution ind the p of M N 4 Br(aq). Is this solution acidic, basic or neutral? K a (N 4+ ) = 5.6 x The solution contains N 4+ ions and Br - ions. N 4+ is the conjugate acid of N 3 (a weak base). N N Acidic solution! Br - is the conjugate base of Br (a strong acid). Br - ions stay fully ionized. Does not change the p. 8

9 p of a Salt Solution p of M N 4 Br? Acidic, basic or neutral? K a (N 4+ ) = 5.6 x N 4+ (aq) + 2 (l) N 3 (aq) (aq) [ ] initial [ ] change - x +x +x [ ] equil x x x K a = 5.6 x = [N 3 ][ 3 + ] = [N 4+ ] (0.132 x) x = 8.57 x 10-6 p = log(8.57 x 10-6 ) = 5.07 Acidic! Salts of Weak Bases and Weak Acids The most difficult. Consider qualitative results. e.g. N 4 (aq) + 2 (l)? N 4+ (aq) + 2 (l) N 3 (aq) (aq) - (aq) + 2 (l) (aq) + - (aq) K a (N 4+ ) = 5.6 x ; K b ( - ) = 1.4 x K a (weak acid) > K b (weak base) The ammonium reaction is more favorable. The solution will be acidic! Acids, Bases and Salts Strong acid + strong base salt solution, p = 7 Strong acid + weak base salt solution, p < 7 Lewis Acids and Bases The most general acid-base definition: A Lewis acid accepts a pair of e - to form a bond. A Lewis base donates a pair of e - to form a bond. Weak acid + strong base salt solution, p > 7 Weak acid + weak base salt solution, p =? Need K s strongest wins A Acid e - pair acceptor..+a B.. B Base e - pair donor Coordinate covalent bond Coordinate covalent bond: a shared e - pair, with both e - donated by the same atom. Lewis Acids and Bases Positive Metal Ions as Lewis Acids Examples: Lewis acid B Lewis acid + + N Lewis base Lewis base B N Metal ions can act as Lewis acids. They have: Missing e -. Empty valence orbitals. Metal ion + Lewis base complex ion Ag + (aq) + 2 :N 3 (aq) [ 3 N:Ag:N 3 ] + (aq) ydroxide ions are Lewis bases. is e - rich (has large electronegativity): Can easily donate an e - pair to a bond. δ- δ+ : - : : 9

10 Positive Metal Ions as Lewis Acids Many metal hydroxides are amphoteric they react with acids and bases. Aluminum hydroxide can act as a Lewis acid: Al() 3 (s) + - (aq) [Al() 4 ] - (aq) Neutral Molecules as Lewis Acids Non-metal oxides act as Lewis acids. attracts e - from any multiple bond. Leaves the other non-metal e - deficient. δ+ S δ- δ- δ+ S δ- δ- δ- δ+ δ- = C = or as a Brønsted-Lowry base: Al() 3 (s) (aq) Al 3+ (aq) (l) Susceptible to attack by Lewis base: - - C = = C = 10